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<title>5.17 copy -- Shallow and deep copy operations</title>
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<h1><a name="SECTION0071700000000000000000">
5.17 <tt class="module">copy</tt> --
         Shallow and deep copy operations</a>
</h1>

<p>
<a name="module-copy"></a>

<p>
This module provides generic (shallow and deep) copying operations.
<a id='l2h-895' xml:id='l2h-895'></a>
<p>
Interface summary:

<p>
<div class="verbatim"><pre>
import copy

x = copy.copy(y)        # make a shallow copy of y
x = copy.deepcopy(y)    # make a deep copy of y
</pre></div>
For module specific errors, <tt class="exception">copy.error</tt> is raised.

<p>
The difference between shallow and deep copying is only relevant for
compound objects (objects that contain other objects, like lists or
class instances):

<p>

<ul>
<li>A <em>shallow copy</em> constructs a new compound object and then (to the
extent possible) inserts <em>references</em> into it to the objects found
in the original.

<p>
</li>
<li>A <em>deep copy</em> constructs a new compound object and then,
recursively, inserts <em>copies</em> into it of the objects found in the
original.

<p>
</li>
</ul>

<p>
Two problems often exist with deep copy operations that don't exist
with shallow copy operations:

<p>

<ul>
<li>Recursive objects (compound objects that, directly or indirectly,
contain a reference to themselves) may cause a recursive loop.

<p>
</li>
<li>Because deep copy copies <em>everything</em> it may copy too much,
e.g., administrative data structures that should be shared even
between copies.

<p>
</li>
</ul>

<p>
The <tt class="function">deepcopy()</tt> function avoids these problems by:

<p>

<ul>
<li>keeping a ``memo'' dictionary of objects already copied during the current
copying pass; and

<p>
</li>
<li>letting user-defined classes override the copying operation or the
set of components copied.

<p>
</li>
</ul>

<p>
This module does not copy types like module, method,
stack trace, stack frame, file, socket, window, array, or any similar
types.  It does ``copy'' functions and classes (shallow and deeply),
by returning the original object unchanged; this is compatible with
the way these are treated by the <tt class="module">pickle</tt> module.

<span class="versionnote">Changed in version 2.5:
Added copying functions.</span>

<p>
Classes can use the same interfaces to control copying that they use
to control pickling.  See the description of module
<tt class="module"><a href="module-pickle.html">pickle</a></tt><a id='l2h-898' xml:id='l2h-898'></a> for information on these
methods.  The <tt class="module">copy</tt> module does not use the
<tt class="module"><a href="module-copyreg.html">copy_reg</a></tt> registration module.

<p>
In order for a class to define its own copy implementation, it can
define special methods <tt class="method">__copy__()</tt> and
<tt class="method">__deepcopy__()</tt>.  The former is called to implement the
shallow copy operation; no additional arguments are passed.  The
latter is called to implement the deep copy operation; it is passed
one argument, the memo dictionary.  If the <tt class="method">__deepcopy__()</tt>
implementation needs to make a deep copy of a component, it should
call the <tt class="function">deepcopy()</tt> function with the component as first
argument and the memo dictionary as second argument.
<a id='l2h-897' xml:id='l2h-897'></a>
<p>
<div class="seealso">
  <p class="heading">See Also:</p>

<dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-pickle.html">pickle</a></tt>:</b>
    <dd>Discussion of the special methods used to
support object state retrieval and restoration.
  </dl>
</div>

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<span class="release-info">Release 2.5.1, documentation updated on 18th April, 2007.</span>
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